diff_drive: add test for controller load failure

This test exercises a controller load failure due to a misnamed
wheel link name in the controller configuration file.
diff_drive: add unit test for bad urdf

This unit test exercises a controller load failure caused by
a wrong wheel geometry. The controller requires that wheels be
modeled by cylinders, while the bad URDF uses spheres.
Merge pull request #34 from po1/test-load-fail

Diff drive: test scenarios to exercise controller load failures